If you forget to logout of Facebook on a public device or suspect an unauthorized third-party login, the first thing to secure your account is to logout of the social network on all your devices. You don’t need to do this manually. Facebook can identify all devices that are currently logged in and allow you to logout from your account on all of them all at once.
1. Click on the down arrow in the top-right corner to open the menu.
2. In the left pane, select Security and Login.
3. Under the Where You’re Logged in section, click See More.
4. At the bottom of the list of sessions, click Log Out Of All Sessions.
5. In the dialogue box, click Log Out to confirm the log out action.